1. Field of the Invention
This invention relates to a component-embedded printed circuit board and a method of forming the same, more particularly to a method involving the use of a releasable film in the process of forming a component-embedding hole in a layered structure of a printed circuit board.
2. Description of the Related Art
FIGS. 1 to 9 illustrate consecutive steps of a conventional method of forming a component-embedded printed circuit board. The conventional method includes the steps of: preparing a copper clad laminate (CCL) 13 that has a core insulator layer 131, a patterned upper foil layer 132 formed on one side of the core insulator layer 131, an upper protection layer 134 of an insulation material covering the upper foil layer 132, a patterned lower foil layer 133 formed on an opposite side of the core insulator layer 131, and a lower protection layer 135 of the insulation material covering the lower foil layer 133 (see FIG. 1), the copper clad laminate 13 having a mounting region 130 with an area substantially equal to a cross-sectional area of an electronic component 18 to be mounted on the mounting region 130 in a subsequent step (see FIG. 8), the upper foil layer 132 having a plurality of spaced apart foil traces 1321, two of the foil traces 1321 being disposed at the mounting region 130 and being exposed from the upper protection layer 134, the copper clad laminate 13 further having a build-up region 130′ surrounding the mounting region 130; preparing a thermosetting adhesive film 11 (see FIG. 2) that is solid at room temperature and that can be softened and cured at an elevated temperature ranging from 150° C. to 220° C.; punching the thermosetting adhesive film 11 under room temperature to form a through-hole 110 therein (see FIG.  2); preparing a resin coated copper (RCC) foil 12 that has a build-up insulator layer 121 and a copper foil layer 122 formed on the build-up insulator layer 121 (see FIG. 3); attaching the punched thermosetting adhesive film 11 to the resin coated copper foil 12 to form a build-up layered structure 10 (see FIG. 3); stacking the build-up layered structure 10 on the copper clad laminate 13 along a stacking direction to form a printed circuit stack 100, such that the through-hole 110 is aligned with the mounting region 130 in the stacking direction (see FIG. 4); heating and pressing the printed circuit stack 100 (see FIG. 5) so as to form the punched thermosetting adhesive film 11 into a cured thermoset film 11′ that adhesively bonds the build-up insulator layer 121 to the build-up region 130′ of the copper clad laminate 13; patterning the copper foil layer 122 to form a plurality of copper traces 1221 on the build-up insulator layer 121 (see FIG. 6); cutting the build-up layered structure 10 through the build-up insulator layer 121 along a cutting path that is aligned with a periphery of the mounting region 130 using a laser beam 150 (see FIG. 7); removing a cut portion of the build-up insulator layer 121 so as to form a component-embedding hole 101 in the build-up layered structure 10 to expose the mounting region 130 (see FIG. 8); and disposing the electronic component 18 in the component-embedding hole 101 and mounting the electronic component 18 on the mounting region 130 to connect electrically with the two foil traces 1321 at the mounting region 130 via a solder material 19 using conventional soldering techniques (see FIG. 9).
It is noted that the thermosetting adhesive film 11 is required to be punched to form the through-hole 110 before the heating and pressing of the printed circuit stack 100. Otherwise, the cured thermoset adhesive film 11′ may be adhesively bonded to the mounting region 130 after the heating and pressing of the printed circuit stack 100 and may not be removed from the mounting region 130 by peeling.
Since the thermosetting adhesive film 11 is softened and is slightly flowable during the heating and pressing operation, an area thereof expands and a thickness thereof becomes thinner after the heating and pressing operation. Hence, because of the expansion, the through-hole 110 in the thermosetting adhesive film 11 is required to have an area that is much greater than that of the mounting region 130, so that the thermosetting adhesive film 11 is prevented from overflowing into the mounting region 130 and cover the two foil traces 1321 at the mounting region 130 during heating and pressing of the thermosetting adhesive film 11. As a consequence, the cured thermoset adhesive film 11′ thus formed may have a curved portion 112 around the through-hole 110, the curved portion 112 having a cross-section with a gradually decreasing thickness (T) (see FIG. 9). The curved portion 112 cooperates with the build-up insulator layer 121 to define a cavity 17 therebetween. As such, residue of an acid solution tends to be trapped in the cavity 17 during subsequent acid washing operations, which can result in chemical attack on the bonding between the build-up insulator layer 121 and the cured thermoset adhesive film 11′ and corrosion to the electronic component 18 and the two foil traces 1321 at the mounting region 130. In addition, a portion 1211 of the build-up insulator layer 121 that is disposed around the component-embedding hole 101 and adjacent to the curved portion 112 is susceptible to mechanical attack during subsequent mounting and soldering operations, which can result in an adverse effect on the performance of the electronic component 18.